Q: How do I enable the kernel's Magic SysRq key sequences, to
be able to do low-level debugging and control of the machine as
described in /usr/src/linux/Documentation/sysrq.txt?

A: In many Linux installations, SysRq (System Request) is
disabled by default. To re-enable it, edit /etc/sysctl.conf to
insert "kernel.sysrq = 1". The "Magic SysRq" keystrokes will
then become available (if support is compiled into the kernel
— see below).

The SysRq key is the same as the Print Screen key.

If X locks, alt-SysRq-r regains control of your keyboard by
taking the console out of Raw mode.

alt-SysRq-k kills everthing on the current virtual
terminal

If your whole machine hangs, you can do

alt-SysRq-s (sync)

alt-SysRq-u (unmount)

alt-SysRq-b (boot)

Umounting and rebooting in this fashion after a hang should
eliminate the necessity to perform an fsck during the
subsequent reboot.

Linux admins enable SysRq ordinarily only when doing
cutting-edge, crash-prone work such as kernel development.
However, you may find it useful if you are reasonably careful
not to invoke it accidentally.

If you compile custom kernels (or suspect SysRq doesn't work
in your Linux distribution's precompiled kernel because of
omitted support for this option), you will want to include the
CONFIG_MAGIC_SYSRQ=y kernel option.